The present invention relates to a hammer drill and more particularly to a mechanism for preventing a striking member of the hammer drill from continuing the non-load striking motion.
A conventional hammer drill includes a striking force transmitting mechanism and a rotational motion transmitting mechanism. The striking force transmitting mechanism converts a rotational motion of a motor into a reciprocative motion of a piston slidably accommodated in a cylinder. A striking member is positioned in the cylinder and is axially offset from the piston via an air chamber. The striking member slides in the cylinder in response to the reciprocative movement of the piston. An intermediate member abuts a working tool detachably held in a tool holder. The striking member strikes the intermediate member. The rotational motion transmitting mechanism transmits the rotational motion of the motor to the working tool via the cylinder and the tool holder. The intermediate member has a large-diameter portion positioned close to the working tool and a small-diameter portion positioned close to the striking member. A rubber or cushion member is provided between the large-diameter portion of the intermediate member and the striking member to absorb the shock to be transmitted from the intermediate member to the piston.
The Japanese Patent Application Laid-Open No. 9-136273 discloses this kind of conventional hammer drill. According to this conventional hammer drill, the cylinder has ventilation holes allowing the air chamber to communicate with the outside of the cylinder. A slide sleeve controls the opening and closing of the ventilation holes of the cylinder in response to a shifting movement of the intermediate member. More specifically, the slide sleeve closes the ventilation holes of the cylinder when the working tool is depressed or pushed against an opponent member to be drilled to enable the hammer drill to perform a striking operation. On the other hand, the slide sleeve opens the ventilation holes of the cylinder when the working tool is not depressed or pushed against the opponent member to bring the hammer drill into an idle or inoperable condition. This is generally referred to as a non-load continuous striking operation preventing mechanism.
The non-load continuous striking operation preventing mechanism includes a slide sleeve which is provided on an outer cylindrical surface of the cylinder and is slidable in the axial direction of the cylinder. A resilient member always urges the slide sleeve toward the working tool. An axial end of the slide sleeve is positioned on an outer surface of the small-diameter portion of the intermediate member. A washer and a damper serving as a cushion member are disposed between a stepped portion of the intermediate member and the axial end of the slide sleeve.
When the working tool is depressed or pushed against the opponent member, the intermediate member shifts toward the striking member relative to the tool holder. The shifting movement of the intermediate member is transmitted via the washer and the damper to the slide sleeve. Thus, the slide sleeve shifts in the axial direction against the urging force of the resilient member. The slide sleeve closes the ventilation holes to enable the hammer drill to perform a striking operation.
After finishing the striking operation, the working tool is released from the depression force having been applied thereon. The intermediate member and the working tool return their home positions by the urging force of the resilient member which is transmitted via the slide sleeve, the washer and the damper to the intermediate member. Thus, the ventilation holes of the cylinder are opened to bring the hammer drill into an idle or inoperable condition.
Furthermore, according to the conventional hammer drill, the tool holder has a slit. The axial end of the slide sleeve, located inside the cylinder, is positioned on an outer surface of the small-diameter portion of the intermediate member. The washer and the damper are disposed between the stepped portion of the intermediate member and the axial end of the slide sleeve. The shock, if caused during a striking operation, is absorbed by the damper before the shock is transmitted from the intermediate member to the striking member, as the slide sleeve is brought into contact with the axial end of the cylinder.
According to the above-described conventional mechanism, one end of the intermediate member is supported by an inner surface of the tool holder having substantially the same radius with that of the large-diameter portion of the intermediate member. On the other hand, the damper and a part of the slide sleeve cooperatively support an axial mid point of the intermediate member. The mid point of the intermediate member is relatively far from the striking member. In other words, there is a significant offset between the closest support point of the intermediate member and the striking member in the axial direction.
The presence of this kind of offset gives adverse influence to the behavior of the reciprocative motion of the intermediate member which is driven by the striking member. More specifically, the intermediate member fluctuates in the radial direction and, as a result, the intermediate member inclines with respect to the axis of the cylinder. The striking force disperses toward circumferential portions of the intermediate member. The striking force transmitting performance deteriorates. The life of the intermediate member and the circumferential peripheral portions will be shortened. The striking force dispersing toward the circumferential portions of the intermediate member will cause vibration in the main body of the hammer drill.
Both of the damper and the slide sleeve need not reciprocate together with the intermediate member. Hence, for the purpose of suppressing a frictional loss, a significant clearance is provided between the outer surface of the intermediate member and the inner surface of the damper or the slide sleeve. Providing such a clearance tends to enlarge the fluctuation of the intermediate member. The inclination of the intermediate member will increase.
In view of the problems of the above-described prior art, the present invention has an object to provide a reliable mechanism for preventing the non-load continuous striking operation of a hammer drill which is capable of suppressing vibration and assuring a long-lasting life.
In order to accomplish this and other related objects, the present invention provides a hammer drill including a crank shaft driven by a motor, a piston engaged with the crank shaft and slidably accommodated in a cylinder to reciprocate in an axial direction of the cylinder. A striking member, axially offset from the piston via an air chamber and slidably accommodated in the cylinder, reciprocates in the axial direction of the cylinder so that a reciprocative motion of the piston is transmitted via the air chamber to the striking member. A tool holder, disposed coaxially with the cylinder, detachably holds a working tool. An intermediate member, slidably accommodated in the tool holder or in the cylinder, reciprocates in the axial direction of the cylinder in response to a reciprocative motion of the striking member to strike the working tool. A rotational motion transmitting mechanism is provided for transmitting a rotational motion of the motor to the working tool. The cylinder has a ventilation hole for allowing the air chamber to communicate with the outside of the cylinder. A slide sleeve is slidable in the axial direction of the cylinder and always urges the intermediate member in a direction opposed to the piston, so that the slide sleeve closes the ventilation hole when the intermediate member is positioned close to the piston while the slide sleeve opens the ventilation hole when the intermediate member is positioned far from the piston. An engaging portion is formed at an axial end of the intermediate member and is engageable with the slide sleeve. The cylinder has an elongated hole with an axial length longer than a shifting distance of the intermediate member. The slide sleeve includes an annular member and a slide member. The annular member has an inner cylindrical wall engageable with the engaging portion of the intermediate member and a projection extending in a radially outward direction passing through the elongated hole beyond an outer surface of the cylinder. And, the slide member is coupled around the outer surface of the cylinder and is slidable in the axial direction of the cylinder. One axial end of the slide member, positioned closer to the working tool, is engaged with the projection of the annular member. And, the other axial end of the slide member is urged toward the working tool by an urging member.
Preferably, the intermediate member includes a large-diameter portion which is positioned close to the working tool and a small-diameter portion which is positioned close to the striking member. A damper, disposed between the large-diameter portion of the intermediate member and the striking member, absorbs an impact force to be transmitted from the intermediate member to the striking member. And, the damper is interposed between a step formed on an inner wall of the tool holder and an axial end of the cylinder.
Preferably, a seal member is provided on an outer surface of the large-diameter portion of the intermediate member so as to be brought into contact with an inner wall of the tool holder.
Preferably, an annular cushion is provided between the engaging portion of the intermediate member and the slide sleeve.